Method and device for producing a workpiece, particularly a shaping tool or a part of a shaping tool

ABSTRACT

A method for producing a workpiece, particularly a shaping tool or a part of a shaping tool, includes the following steps:
         providing a heat-resistant mold ( 2 ) with a first molded part ( 2   a ) and at least a second molded part ( 2   b ) in a chamber that can be evacuated ( 1 );   filling a metal-containing material into the heat-resistant mold ( 2 );   producing a vacuum in the chamber that can be evacuated ( 1 );   heating the metal-containing material;   compressing the heated metal-containing material in the heat-resistant mold ( 2 ) by hot pressing under vacuum conditions. The present invention further relates to a device for producing a workpiece, particularly a shaping tool or a part of a shaping tool.

The present invention relates to a method for producing a workpiece, particularly a shaping tool or a part of a shaping tool. Furthermore, the present invention relates to a device for producing a workpiece, particularly a shaping tool or a part of a shaping tool.

Methods and devices of the type mentioned at the beginning for producing a workpiece, particularly a shaping tool or a part of a shaping tool, are known in various embodiments from the prior art. Two of the most customary methods for producing, for example, shaping tools or parts of shaping tools are the casting and the (mechanical) machining of forged blocks.

During the casting of a shaping tool or of a part of a shaping tool, an alloy having the desired composition is first of all melted. The melt is then cast into a casting mold, the shape of which is already close to the desired final shape of the shaping tool or part of a shaping tool. When the alloy melt in the casting mold has solidified, it is first of all roughly machined, then heat-treated and subsequently finely finished.

During the machining of forged blocks, the liquid melt is first of all cast to form a bar. When the bar has solidified, said bar is extracted, then heated up again and subsequently—customarily in a plurality of steps—forged to form rods or blocks. The rods or blocks obtained in this manner are then heated up again in order to initiate a curing process which simplifies the subsequent finishing operation. When the rods have cured, they are customarily cut into blocks having the desired dimensions. Said blocks are then brought in a rough machining step into a shape which is already close to the final shape of the shaping tool or of the part of a shaping tool. This is followed, as in the above-described casting method, by a heat treatment and a fine machining operation in order to end the production of the shaping tool or of the part of a shaping tool. The machining of the rods or blocks is relatively complicated, and therefore the machining costs in this method are much higher than the material costs.

If the two above-described production methods are considered somewhat more precisely, the question is posed why shaping tools or parts of shaping tools are produced from forged blocks at all. In fact, casting methods for producing shaping tools or parts of shaping tools appear to be more expedient at first sight, since said casting methods are considerably more cost-effective than the machining of forged blocks. On the other hand, the mechanical properties of the shaping tools or parts of shaping tools obtained by the two above-described methods differ greatly. It is the resulting toughness/ductility of the shaping tools or parts of shaping tools which makes the considerably more complex in terms of method and therefore substantially more expensive machining of forged blocks advantageous. This is because, during the forging process, the relatively brittle carbide networks of the cast part are destroyed and, as a result, the toughness/ductility may even be doubled. If the parts are relatively large (greater than 600 mm for most cold work tool steels and 200 mm for most rapid work steels), this effect is not achieved in the core region of the block or of the rod where the relatively brittle cast part structure is retained.

If even greater toughness/ductility is desired for the shaping tool or part of a shaping tool, an even more complex method is frequently used. This involves “powder metallurgy” (PM for short). In this case, the melt is not placed into a casting block but rather into an atomizer which customarily has at least one nozzle in which a gas is burnt toward the liquid flow of metal, resulting in the metal being evaporated to form small, substantially spherical particles (powder). The metal powder obtained in this manner is subsequently placed into steel containers which are of substantially cylindrical shape. The steel containers are then evacuated in order to obtain a certain amount of vacuum and inserted into a hot isostatic pressing device in which high temperatures and pressures are generated in order to deform the steel containers and to compact the metal powder such that a bar is obtained. When the container material has been removed, a bar remains, which bar can be forged and can be subjected to all the machining steps which have been explained in more detail above in the description of the method “machining of forged blocks”. Although this production method increases the production costs, the toughness/ductility which can be achieved is much higher than blocks which have been melted and forged in a conventional manner. This can be achieved in particular if the alloy has a relatively high content of a brittle ceramic phase.

Although the forging, on the one hand, increases the toughness/ductility of the shaping tool or part of a shaping tool, on the other hand, a number of undesirable properties, of which the most important is anisotropy, are induced. This is because the forging creates a material structure which leads to different properties in the forging direction in comparison to the directions running transversely with respect to the forging direction. Said anisotropy can be disadvantageous in particular during the heat treatment of the shaping tool or part of a shaping tool, since said anisotropy results in a distortion of the workpiece, and therefore more material has to be left over for the final machining operation.

A further method for producing a shaping tool or a part of a shaping tool is the sintering of a metal powder. In this case, a metal powder or, alternatively, a mixture of a plurality of metal powders is pressed in order to obtain a body having the desired geometrical shape and having a suitable structure such that it can subsequently be further machined. A body of this type is frequently also referred to as a “green body”. The green body is then sintered at a high temperature for a sufficiently long period of time in order to promote diffusion bonding. If a high density is desired, a final, hot-isostatic pressing step is carried out. Various variants of a method of this type are disclosed, for example, in DE 198 252 23 C2, WO 02/20863 A1, DE 195 08 959 C2, DE 197 52 505 C1, DE 698 148 96 T2, EP 1 281 461 A1 and EP 0 919 635 A2.

There has recently been a sharp rise in the industrial demand for workpieces and components having tailored properties. New concepts, such as, for example, the intelligent production of tools, have therefore been developed. For many purposes, the simplest way of obtaining a component with tailored properties is to use a shaping tool or a part of a shaping tool having appropriately graduated properties.

For example, DE 195 089 59 C2 discloses a molded body made of a ceramic, powder-metallurgical or composite material. Furthermore, a method for producing a molded body of this type is disclosed in the abovementioned reference. The material composition and/or the structure are/is changed within the molded body in one, two or all three directions in space. The changes may be continuous or discontinuous. In order to produce the molded body, one or more starting powders are processed to form one or more moldable compounds. Said moldable compound/compounds is/are processed continuously or discontinuously in one, two or all three directions in space to form a molded body and is/are subsequently cured, wherein the moldable compound or compounds is or are applied in relation to the graduation of properties to be obtained at the end.

It is furthermore known from the prior art to use concrete (despite the very low strength and toughness thereof in comparison to other materials) to produce shaping tools. For example, DE 699 08 273 T2 describes a pressing tool which consists primarily of concrete.

The international patent application WO 2006/056621 A2 describes a method for producing a shaping tool by superplastic deformation or by hot compaction of a metallic powder in a cement mold. The shaping tool properties obtained by the hot compaction of the powder are not particularly good with particular regard to the strength and toughness/ductility. The reason for this resides in particular in the surface oxidation of the metallic powder, the surface oxidation preventing the optimum compaction of said powder. The present invention is based on the object of providing a method for producing a workpiece, particularly a shaping tool or a part of a shaping tool, which method permits production of a workpiece in a manner close to the final shape, the workpiece having an advantageous combination of high strength and at the same time favorable toughness and ductility properties. Furthermore, the present invention is based on the object of providing a device for producing a corresponding workpiece, particularly a shaping tool or a part of a shaping tool.

This object is achieved with regard to the method by a method having the features of claim 1. A method having the features of claim 4 provides a further solution according to the invention. With regard to the device, the object on which the present invention is based is achieved by a device having the features of claim 10. The dependent claims relate to advantageous developments of the invention.

According to claim 1, a method according to the invention for producing a workpiece, particularly a shaping tool or a part of a shaping tool, comprises the following steps:

-   -   providing a heat-resistant mold having a first mold part and at         least one second mold part in an evacuable chamber, wherein the         heat-resistant mold consists of concrete, cement or mortar         admixed with at least one ceramic material,     -   placing a metal-containing material into the heat-resistant         mold,     -   generating a vacuum in the evacuable chamber,     -   heating the metal-containing material,     -   compacting the heated, metal-containing material in the         heat-resistant mold by hot pressing under vacuum conditions.

According to claim 1, the workpieces, such as, for example, shaping tools or parts of shaping tools, are produced in a heat-resistant mold under vacuum conditions by compaction of the heated, metal-containing material which may be present, for example, as a solid, metal-containing body. The workpiece is produced in an evacuable chamber in which the vacuum can be generated and in which, if appropriate, an inert gas atmosphere and/or a reduction gas atmosphere can also be generated. The method according to the invention permits the production in a particularly advantageous manner of locally isotropic workpieces (for example, shaping tools or parts of shaping tools) which are close to the final shape, as can be obtained by the casting methods known from the prior art, with mechanical properties, as can be obtained by the known powder metallurgy methods. By means of the generation of a vacuum, in particular the oxygen content in the residual gas within the evacuable chamber can be reduced such that oxygen contamination of the surface of the metal-containing material is substantially prevented, but can be at least considerably reduced, and therefore workpieces, such as, for example, shaping tools or parts of shaping tools, of particularly high quality can be produced.

In order to further reduce the oxygen content within the evacuable chamber after the metal-containing material has been placed into the heat-resistant mold, it is proposed, in a particularly advantageous embodiment, that a number of flushings of the evacuable chamber with a reduction gas and/or an inert gas is carried out before the vacuum is generated in the evacuable chamber. A vacuum can advantageously be generated in the evacuable chamber (at least temporarily) between two flushings with the inert gas or reduction gas. The vacuum does not need to be a high vacuum. The residual gas level within the evacuable chamber is relatively low because of the vacuum generation following the flushings.

In a particularly preferred embodiment, it is proposed that a high vacuum is generated in the evacuable chamber and the hot pressing of the heated metal-containing material is carried out under high vacuum conditions. The pressure range of the high vacuum which is generated within the evacuable chamber advantageously lies within an order of magnitude of between approximately 10⁻³ and approximately 10⁻⁷ mbar. The production of a high vacuum enables in particular the oxygen content in the residual gas within the vacuum chamber to be further reduced such that oxygen contamination of the surface of the metal-containing material can be further reduced.

According to claim 4, an alternative method according to the invention for producing a workpiece, shaping tool or a part of a shaping tool comprises the following steps:

-   -   providing a heat-resistant mold having a first mold part and at         least one second mold part in an evacuable chamber, wherein the         heat-resistant mold consists of concrete, cement or mortar         admixed with at least one ceramic material,     -   placing a metal-containing material into the heat-resistant         mold,     -   generating a vacuum in the evacuable chamber and maintaining the         vacuum for a period of time t_(vacuum),     -   generating an inert gas atmosphere or reduction gas atmosphere         in the evacuable chamber after expiry of the period of time         t_(vacuum),     -   heating the metal-containing material,     -   compacting the heated, metal-containing material in the         heat-resistant mold by hot pressing in the inert gas atmosphere         or reduction gas atmosphere.

In said alternative solution according to the invention, the compaction of the heated, metal-containing material in the heat-resistant mold by means of hot pressing is therefore not carried out under vacuum conditions but rather in an inert gas atmosphere or reduction gas atmosphere. As a result, oxygen contamination of the metal-containing material can likewise be effectively prevented. This method according to the invention likewise permits in a particularly advantageous manner the production of locally isotropic shaping tools or parts of shaping tools close to the final shape, as can be obtained by the casting method, with strength properties, as can be obtained by a powder metallurgy method.

One advantage of the method according to the invention presented here is that the workpiece tolerances which can be achieved are so small that a customarily following rough machining step (for example, by means of metal-cutting machining methods) of the workpiece can likewise be eliminated. A workpiece (shaping tool or part of a shaping tool) produced by the methods presented here preferably has an impact value of more than 50 J/cm² while the degree of hardness is preferably greater than 58 HRC.

In a particularly preferred embodiment, it is proposed that the hot pressing is carried out at a constant rate of expansion. The rate of expansion can be kept constant, for example, via a change in the advancing speed of a metal cylinder, by means of which a pressure is exerted on the mold and therefore on the metal-containing material.

It can be provided, in an advantageous embodiment, that the metal-containing material is placed in the form of at least one layer of a metal-containing powder or of a metal-containing powder mixture into the heat-resistant mold. The method then in particular comprises the step of hot pressing the metal-containing powder or the metal-containing powder mixture in the heat-resistant mold to which preferably only a small amount of water is admixed. The metal-containing powder used for producing the workpiece can consist entirely of one material. The metal-containing powder may also be, for example, a mixture of a metal powder with ceramic particles, wherein the ceramic particles, for their part, may have a coating.

For numerous applications, the possibility of using mixtures of different metal-containing powders or a plurality of layers or regions of different metal-containing powders and of inputting said powders or layers into the heat-resistant mold may be particularly advantageous. It is therefore proposed, in a particularly advantageous embodiment, that, when the metal-containing powder or the metal-containing powder mixture is placed in, at least two layers or regions having a different chemical composition are produced. As a result, it is possible in a particularly advantageous manner to produce workpieces (in particular shaping tools or parts of shaping tools) having tailored mechanical and/or physical properties which may also be graduated within the volume of said workpieces. In other words, this therefore also enables the production of workpieces which have different mechanical and/or physical properties in one, two or else all three directions in space within the volume thereof. The graduations of properties may be continuous or else discontinuous.

Of particular interest in this connection is also the possibility of mixing a metal powder with comparatively hard, coated or uncoated particles. This can give rise to a better ratio between the toughness/ductility and the wear resistance. However, the coated particles which can be mixed with the metal-containing powder are relatively expensive as a rule. The methods described here permit the use of particles of this type without causing a deterioration in the intrinsic properties thereof and furthermore permit the use of only the minimum required quantity of said particles. This is because the particles can be placed only in those regions of the shaping tool or part of a shaping tool where said particles are actually required, and therefore the production costs for the workpiece can be kept as low as possible. Some coated particles necessitate the control of certain diffusion parameters (in particular temperature and time) during the process in order to avoid a deterioration in the intrinsic properties or to obtain optimum properties, wherein the processes presented here are particularly suitable therefor. There is in particular the possibility of introducing different types of metal-containing powders or metal-containing powder mixtures into different working and carrier zones of a shaping tool or of a part of a shaping tool in order thereby to obtain tailored properties for the tool.

In a particularly preferred embodiment, it is proposed that the heated, metal-containing material is compacted in a superplastic state of the material. The metal-containing material is preferably heated up comparatively slowly in the heat-resistant mold in order to achieve the superplastic state. The superplastic state of a metal-containing material is customarily achieved (depending on the material and depending on the rate of expansion) at a temperature of approximately 800° C. to approximately 1050° C. The compaction of the heated, metal-containing material in the superplastic state is advantageous in particular if the material is present in the form of a body, the geometry of which is preshaped. This is the case, for example, for a precompacted, dimensionally stable green compact. If the metal-containing material is present in powder form, the hot pressing can also be carried out in a non-superplastic state although the compaction in the superplastic state is also particularly advantageous here.

In order to be able to produce the greatest possible density within the workpiece particularly in the case of a metal-containing material present in powder form, a particularly preferred variant of the method provides that the metal-containing material is heated up further after reaching the superplastic state to the diffusion acceleration temperature thereof. Said diffusion acceleration temperature is dependent on the alloy and, for example in the case of tool steel, is of an order of magnitude of approximately 1150° C. By contrast, alloys made of molybdenum have a higher diffusion acceleration temperature of above 1800° C. and alloys made of copper have a diffusion acceleration temperature which is lower than 900° C. The diffusion acceleration temperature is preferably maintained for a relatively long period of time, customarily for a period of time of more than 30 minutes. The maintaining time depends in particular on the diffusion acceleration temperature and the pressure exerted. This may be, if appropriate, a number of hours or even a number of days.

In a further advantageous embodiment, it is proposed that the metal-containing material is at least partially melted and is compacted in an at least partially liquid state. The metal-containing material does not need to be completely melted. There is the possibility, for example, of only one phase of the metal-containing material being melted before the compaction. This embodiment of the method may be advantageous for a number of application purposes.

So that a metal-containing powder or a metal-containing powder mixture achieves the greatest possible density thereof during the hot pressing operation, in particular the following process conditions have to be observed:

-   -   a) sufficient time and a sufficiently high temperature such that         the diffusion process can take place;     -   b) a sufficiently high pressure so that the metal-containing         powder can flow and fill the voids (a region of the superplastic         deformation of the metal-containing powder is particularly         advantageous in this case, as already mentioned above);     -   c) the absence of oxygen in order to avoid surface oxidation of         the metal-containing powder.

The pressure which is exerted on the heated, metal-containing material during the hot pressing operation (for example, by means of a metal cylinder) is preferably greater than 20 MPa. The pressure during the compaction of the metal-containing material may be in particular between approximately 20 MPa and approximately 250 MPa—depending on the load-bearing capacity of the mold material.

In a further advantageous development of the method, before the metal-containing material is heated up, a cold pressing step can be carried out. The latter is particularly advantageous in particular when a metal-containing powder or a metal-containing powder mixture is used. The cold pressing step enables the porosity of the metal-containing powder or of the metal-containing powder mixture to be closed so as to leave as little interconnected porosity as possible in the material. After the cold pressing step, the evacuable chamber can be flushed, if appropriate, with a reduction atmosphere before continuing with the other method steps. In an alternative variant, the metal-containing powder or the metal-containing powder mixture can also be input into a controlled atmosphere environment. Irrespective of which process is pursued, it is important to avoid as far as possible the presence of oxygen between powder grains.

In a particularly preferred embodiment, it is provided that the process heat is removed in a targeted manner, preferably by means of a cooling device, after the compaction of the metal-containing material. One purpose of such a targeted removal of heat can involve accelerating the production method as a whole. Furthermore, the microstructural properties of the workpiece can be set by the targeted removal of heat. In this case, it is particularly advantageous if the pressure exerted on the metal-containing material is maintained during the targeted removal of the process heat (cooling phase). By means of this measure, deviations in geometry, in particular shrinkages of the workpiece, can be substantially prevented.

In a further advantageous embodiment, it is proposed that the step of compacting the metal-containing material or of removing the process heat is followed by at least one finishing step. This at least one finishing step may in particular comprise the carrying out of a fine machining process and/or hard machining process. For example, grinding processes, high speed milling processes or thermally assisted laser machining processes may be used. The at least one finishing step is not carried out under vacuum conditions or under a gas atmosphere.

In summary, the methods presented here for producing a workpiece, in particular a shaping tool or a part of a shaping tool, permit tailoring of functionalities which can be derived directly from the metallurgical and microstructural composition of the metal-containing material used (in particular tool steel).

Functionalities of this type are, for example,

-   -   wear resistance,     -   heat conductivity,     -   electric conductivity,     -   resistance under thermal and/or mechanical stressing,     -   sensor and actuator functionalities on the basis of         piezo-electric effects, a shape memory effect or the absorption         of electromagnetic waves.

There may be different motivations for such a locally variable distribution of functionalities.

1. Integral Temperature Management

In the sphere of forming sheet-metal materials, the technology involved in press hardening, shape hardening or profile hardening is of increasing interest for the production of components of this type which, during the use thereof, have to satisfy extremely exacting requirements with regard to safety and energy efficiency. Processes of this type are characterized, firstly, by the use of a significantly increased shape changing capacity at high process temperatures and, secondly, by the thermal treatment which immediately follows the shaping or takes place at the same time therewith. If a thermal treatment of this type involves hardening, it is desirable for technological reasons, but also for process productivity reasons, on the one hand, to rapidly remove the heat stored in the workpiece but, on the other hand, premature hardening may profoundly obstruct the further shaping by reducing the shape changing capacity. The consequences range from increased shaping forces up to the component failing due to cracking. The risk of a premature removal of heat occurs in the region of the sheet-metal holder during the press hardening operation since the first contact of tool-workpiece occurs here even before the beginning of the actual forming process and therefore long before the final contour of the workpiece is achieved. The premature cooling associated therewith can be avoided or the effect thereof can be reduced by locally present, low heat conductivities being provided in this region for the tool material. By contrast, due to the time-delayed contact between the tool and the workpiece in the remaining regions, correspondingly higher heat conductivity is entirely desired here.

2. Time- and Locally Variable Temperature Management

If it is now the aim, by rendering the thermal process conditions variable with the effect of locally and time-variable thermomechanical process conditions, to produce components which have complex microstructural property profiles, then process strategies of this type give rise to extremely complex requirement profiles for the tool. Locally variable heat conductivities are merely one embodiment feature in this case; by means of the use of locally variable electric and piezoelectric effects, even shape memory effects as far as the absorption of electromagnetic waves, other features can consist in assisting said flexibility with regard to sensor and actuator tasks with the effect of a specific setting of local thermal and/or mechanical process conditions.

According to claim 10, a device according to the invention for producing a workpiece, particularly a shaping tool or a part of a shaping tool, comprises:

-   -   an evacuable chamber,     -   a heat-resistant mold which is accommodated in the evacuable         chamber and has a first mold part and at least one second mold         part which form a mold cavity, wherein a metal-containing         material, in particular a metal-containing powder or a         metal-containing powder mixture, can be placed into the mold         cavity, wherein the heat-resistant mold consists of concrete,         cement or mortar admixed with at least one ceramic material,     -   means for generating a vacuum in the evacuable chamber,     -   means for heating the metal-containing material in the         heat-resistant mold,     -   means for compacting the heated, metal-containing material in         the heat-resistant mold by hot pressing.

The device according to the invention is suitable in particular for carrying out a method as claimed in one of claims 1 to 9 such that workpieces, such as, for example, shaping tools or parts of shaping tools, can be produced with the above-described advantageous properties. The heat-resistant mold which is used for producing the workpiece is intended to be suitable, with respect to the mechanical configuration thereof, to withstand the pressure which is required in order to allow a metal-containing powder or a metal-containing powder mixture to flow. The pressure acting on the metal-containing material during the compaction operation can be between approximately 20 MPa and approximately 250 MPa—depending on the load-bearing capacity of the mold material.

In a particularly advantageous embodiment, it is provided that the device furthermore has means for generating an inert gas atmosphere and/or a reduction gas atmosphere. With such a configuration of the device, the compaction of the metal-containing material can be carried out in an inert gas atmosphere or a reduction gas atmosphere. As a result, oxygen contamination of the surface of the metal-containing material can be prevented.

In a particularly preferred embodiment, the heat-resistant mold can be a ceramic-containing and/or graphite-containing mold. The pressures acting on the mold material from which the heat-resistant mold is produced are generally greater than 20 MPa, frequently also greater than 30 MPa to 40 MPa such that concrete, mortar or cement with a small admixture of water and with a content of at least one ceramic material are particularly advantageous as materials for producing the heat-resistant mold. In this case, Al₂O₃, zirconium oxide, silicon carbide or Si0 ₂ are the preferred additional materials for producing the heat-resistant mold. The concrete, cement or mortar can preferably have a content of at least 40%, preferably of at least 60%, in particular of at least 80% Al₂O₃. It can also be provided, for example, that the concrete, cement or mortar has a strength which is greater than 150 MPa (preferably greater than 200 MPa).

In a particularly advantageous embodiment, the means for heating the metal-containing material can comprise at least one heating element which can be embedded, for example, into the heat-resistant mold. The at least one heating element preferably extends in the circumferential direction of at least one of the mold parts (preferably at a distance of approximately 10 to 20 mm from the mold cavity) such that uniform heating-up of the metal-containing material can be achieved. The at least one heating element can consist, for example, of an Ni—Cr resistance wire or of an Fe—Cr—Al resistance wire. Other resistance heating wires which can consist, for example, of molybdenum or tungsten can likewise be used. An inductively operating heating element can also be used.

Furthermore, in a preferred embodiment, at least one cooling device can be provided, which cooling device can advantageously likewise be embedded into the heat-resistant mold and is suitable for cooling the metal-containing material within the heat-resistant mold in a targeted manner. As a result, a cooling option for the metal-containing material placed into the heat-resistant mold can additionally be provided. The cooling device can comprise, for example, a number of cavities which are introduced in a defined manner into the heat-resistant mold during production. A liquid or gaseous cooling fluid can flow through said cavities and can be conveyed by means of a supply device or the like in order to be able to cool the metal-containing material in the heat-resistant mold in a targeted manner after the compaction operation.

The cooling device may comprise, for example, at least one tube which is embedded into the heat-resistant mold and through which a liquid or gaseous cooling fluid can circulate. Furthermore, the evacuable chamber may also be flooded with a gaseous cooling fluid (for example, with nitrogen or argon). The gaseous cooling fluid can preferably flow out of a pressure tank or a pressurized gas cylinder into the cavities, the tube or the evacuable chamber, since the gas cools further upon expansion. There is in particular the possibility of the cooling device forming a cooling circuit, within which the cooling fluid can circulate and within which, for example, a heat exchanger or a compression stage can be provided.

Furthermore, at least one temperature detection means and regulating means can be provided in order to regulate the temperature of the metal-containing material in the mold.

In order to produce the heat-resistant mold, first of all a model having the desired geometry of the workpiece (for example a shaping tool or part of a shaping tool) is produced. Said mold model can be produced from different materials (for example, from polystyrene, polypropylene, wood or aluminum). Numerous other thermoplastics, metals or even ceramic materials can be used to produce the mold model. In order to obtain the mold model, use may be made of conventional method techniques or else of “rapid-prototyping techniques” (for example, mechanical machining, stereo-lithography, three-dimensional wax impression, casting etc.).

With the mold model obtained in this manner, the mold can be produced, for example by casting of the heat-resistant mold material, in particular if the mold material contains a powder or powder mixture, concrete, mortar or the like. If the mold is produced in this manner, it is very simple to embed at least one heating element (in particular a resistance heating element or an induction heating element), a cooling element and, if appropriate, also temperature detection means into the mold. If the mold is produced by a three-dimensional ceramic printing technique or by a comparable technique which permits the heat-resistant mold to be obtained directly—i.e. without further intermediate steps, a corresponding mold model does not have to be produced. The same applies if the heat-resistant mold is obtained by direct mechanical machining of a solid block of a heat-resistant mold material.

If the heat-resistant mold is produced, for example, from concrete, the concrete is placed into the mold model together with a small admixture of water and preferably an admixture of a ceramic material. In order to avoid pore formation and to fill even complicated geometries with the mold material, the mold model should be filled as rapidly as possible. The mold is subsequently cured at a high temperature (for example, approximately 1200° C.) such that the residual moisture can escape from the concrete. There is furthermore the option of vibrating the mold model as the mold material is being placed therein, for example on a vibrating table or the like. It has been shown that the porosity of the mold can be substantially reduced as a result.

After the mold has been produced in the above-described manner and has been provided in the evacuable chamber, the mold cavity can be at least partially filled with the metallic material, in particular with a metal-containing powder or with a metal-containing powder mixture. This is then followed by the remaining process steps to produce the workpiece.

In a particularly preferred embodiment, it is proposed that at least sections of the surface of the mold cavity of the heat-resistant mold have a ceramic layer and/or a releasing-agent and lubricating layer. The ceramic layer may be, for example, an oxide layer (for example consisting of zirconium oxide) or a carbide layer (for example consisting of silicon carbide). Any other ceramic material which does not react with hot metal can likewise be used. The releasing-agent and lubricating layer can consist, for example, of graphite, molybdenum disulfide, sulfur, phosphorus, boron nitride, mica or of another material which can withstand the relatively high process temperatures. It is likewise highly desirable for the mold material used to have relatively low heat conductivity so that said mold material can serve as an insulator between the heating-up zone, in which the metal-containing powder or the metal-containing powder mixture is heated, and the exterior of the mold, in particular if, in an advantageous embodiment, the heat-resistant mold has a prestressed reinforcing ring made of metal. A prestressed reinforcing ring of this type can produce pressure stresses in the mold in order to compensate for tensile stresses which arise during the compaction of the heated, metal-containing material.

In order to avoid the metal-containing material reacting with the mold material of the heat-resistant mold, in a particularly advantageous embodiment, at least sections of the surface of the mold cavity can have a dye layer or a dispersion layer. The surface of the mold cavity can be rendered chemically more inert by the application of a dye or dispersion. Lubricants may also be used for this purpose. It may also be advantageous to increase the emissive power of the surface of the ceramic mold in order to configure the process to be more efficient in terms of energy and to keep the heat at the location where it is required. The active material of the dye or of the dispersion may be, for example, zirconium oxide, boron nitride or molybdenum disulfide or may comprise other components on the basis of graphite, phosphorus or sulfide (to mention just some components).

In order to increase in particular the shear strength of the heat-resistant mold, it can be provided, in a preferred embodiment, that the heat-resistant mold is reinforced with metal particles and/or metal rods and/or metal wires and/or metal wire fabrics. Iron or steel can be used as the material. However, for high temperature applications, particularly heat-resistant metals, such as, for example, tungsten or molybdenum and the alloys thereof and also alloys on the basis of nickel or cobalt may be more advantageous. Furthermore, in order to reinforce the heat-resistant mold, textile fibers and/or polymer fibers and/or ceramic fibers and/or glass fibers and/or long fiber fabrics of said materials may also be used.

The means for compacting the metal-containing material may in particular comprise a metal cylinder which is operatively connected to the second mold part of the heat-resistant mold. During the operation of the device, the metal cylinder can exert a sufficiently high pressure on the heat-resistant mold or on part of the heat-resistant mold in order thereby to compact the metal-containing material in the mold.

Further features and advantages of the present invention will become clear using the description below of preferred exemplary embodiments with reference to the attached figure, in which

-   -   FIG. 1 shows a schematically highly simplified illustration of a         device according to a preferred exemplary embodiment of the         present invention, which device is suitable for carrying out a         method for producing a workpiece, particularly a shaping tool or         a part of a shaping tool.

A device which is suitable for carrying out a method for producing a workpiece, particularly a shaping tool or a part of a shaping tool, comprises an evacuable chamber 1 with a vacuum system, by means of which a vacuum, preferably a high vacuum of an order of magnitude of between 10 ⁻³ and 10 ⁻⁷ mbar, can be generated in the interior of the evacuable chamber. The vacuum system may comprise, for example, a vane-type rotary pump and a turbomolecular pump connected thereto. In this case, the vane-type rotary pump generates a fore-vacuum for the turbomolecular pump. Furthermore, pressure sensor means are provided so that the pressure within the evacuable chamber 1 can be measured and continuously monitored.

The device furthermore has a heat-resistant mold 2 which may be, for example, ceramic-containing and/or graphite-containing and comprises a first (lower) mold part 2 a with a mold cavity, and a second (upper) mold part 2 b guided movably relative to the first mold part 2 a. It is seen that the inside diameter of the first mold part 2 a is greater than the outside diameter of the second mold part 2 b, and therefore the second mold part 2 b can be inserted into the mold cavity of the first mold part 2 a. The two heat-resistant mold parts 2 a, 2 b are preferably produced from concrete and a ceramic material (for example, Al₂O₃) and have only a small admixture of water.

A heating element 3 is embedded into the first mold part 2 a so that the first mold part 2 a can be heated up as the method is being carried out. The distance of the heating element 3 from the inner surface of the mold cavity of the first mold part 2 a is preferably approximately 10 to 20 mm. The heating element 3 is required in order to achieve the temperature necessary for producing the workpiece. It is advantageous in this case if, as in the exemplary embodiment shown here, the heating element 3 is embedded directly into the heat-resistant mold 2. The heating element 3 may be, for example, a resistance heating element or an induction heating element, the last-mentioned variant being more advantageous because of having shorter heating-up times and better insulation, even though it is somewhat more difficult to calibrate. Furthermore, a cooling device (not shown explicitly in FIG. 1) may be provided, by means of which the metal-containing workpiece in the mold 2 can also be cooled.

FIG. 1 shows, by way of example, one possible position of the heating element 3 within the first mold part 2 a of the heat-resistant mold 2. As an alternative thereto, the first mold part 2 a may also be constructed in a modular manner and may have, for example, an inner contact layer adjoined by the heating element 3 and finally an insulation shield. Furthermore, in the exemplary embodiment shown here, a temperature detection means 4, which may in particular comprise a conventional thermocouple, is embedded into the first mold part 2 a. As a result, the process temperature can be continuously monitored as the method is being carried out. By means of the provision of the at least one heating element 3, if appropriate a cooling device, the temperature detection means 4 and a regulating device, the process temperature can be very exactly regulated as the method is being carried out.

Furthermore, the device has a metal cylinder 5 by means of which a pressure can be exerted in the arrow direction on the second (upper) mold part 2 b. The surface of the mold cavity of the heat-resistant mold 2 can advantageously be coated with a ceramic layer and/or with a releasing-agent and lubricating layer. The ceramic layer may be, for example, an oxide layer (for example consisting of zirconium oxide) or a carbide layer (for example consisting of silicon carbide). Any other ceramic material which does not react with the hot metal within the mold cavity can likewise be used. The releasing-agent and lubricating layer may consist, for example, of graphite, molybdenum disulfide, sulfur, phosphorus, boron nitride, mica or of another material which can withstand the high process temperatures. Furthermore, there is the option of using a glass powder as the releasing agent. Glass has the advantage that, at high temperatures, it forms a glass separating layer which can effectively prevent disadvantageous surface reactions with the surrounding atmosphere.

A metal-containing material in the form of a metal-containing, solid body or at least one layer or one region of a metal-containing powder or of a metal-containing powder mixture from which the workpiece is intended to be produced is placed into the mold cavity of the first mold part 2 a and heated therein, according to a first exemplary embodiment, under high vacuum conditions with the aid of the at least one heating element 3. In order for the metal-containing powder or the metal-containing powder mixture to be compacted as rapidly as possible, the generation of a high vacuum in the interior of the evacuable chamber 1 during the compaction operation is particularly advantageous. As a result of the fact that the heating is carried out under high vacuum conditions in the evacuable chamber 1, oxygen contamination of the metal-containing material can be effectively prevented, but at least considerably reduced. This is of particular importance in particular in the event of use of a metal-containing powder or a metal-containing powder mixture, in order to obtain optimum tool properties.

With most mold materials from which the mold can be produced, the generation of a high vacuum is quite difficult since said mold materials have an outgassing tendency in particular at higher temperatures. It is furthermore important as far as possible not to have any oxygen which affects the quality of the powder surface and prevents the complete compaction and diffusion welding of the metal-containing powder or powder mixture. One option for obtaining better process conditions consists in curing the heat-resistant mold 2 in a reduction gas atmosphere in order thereby to ensure that voids within the mold material are filled with the reduction gas atmosphere. As an alternative thereto, the heat-resistant mold 2 can be heated up in the evacuable chamber 1 prior to the metal-containing material being placed therein, and then a vacuum can be generated and the chamber 1 can subsequently be filled with a reduction atmosphere in order to fill the voids within the mold material.

In order further to reduce the oxygen content in the evacuable chamber 1 and therefore further to improve the process conditions, there is furthermore the option of carrying out a plurality of flushings of the evacuable chamber 1 with a reduction gas atmosphere and/or an inert gas atmosphere before, finally, the high vacuum is generated in the evacuable chamber 1. A vacuum can advantageously be generated, at least temporarily, in the evacuable chamber 1 between two flushings.

According to a second exemplary embodiment, after the metal-containing material is placed into the heat-resistant mold, first of all a vacuum is generated in the evacuable chamber 1 and maintained for a certain period of time t_(vacuum). After expiry of the period of time t_(vacuum), an inert gas atmosphere or reduction gas atmosphere is generated in the evacuable chamber 1 and the metal-containing material is heated. The heated, metal-containing material in the heat-resistant mold 2 is then compacted by hot pressing in the inert gas atmosphere or reduction gas atmosphere.

In both exemplary embodiments, the metal-containing material is heated up after being placed into the mold cavity of the first mold part 2 a of the heat-resistant mold 2 and, in the process, is optionally transferred into a superplastic state which is achieved (depending on the material) at temperatures of between approximately 800° C. and approximately 1050° C. If the metal-containing material is present in the form of a metal-containing body, the hot pressing preferably takes place in said superplastic state. The hot pressing advantageously takes place at a constant rate of expansion and a constant advancing speed of the metal cylinder 5. The pressure which is generated during the hot pressing by the metal cylinder 5 and acts via the second mold part 2 b on the metal-containing material within the first mold part 2 a can be between approximately 20 MPa and approximately 250 MPa. The pressure here can act on the metal-containing material within the heat-resistant mold 2 continuously or only in phases.

If the metal-containing material is present in the form of a metal-containing powder or a metal-containing powder mixture, the hot pressing can also take place under non-superplastic conditions. However, it is particularly advantageous to compact the metal-containing powder or the metal-containing powder mixture by hot pressing in the superplastic state. The metal-containing powder or the metal-containing powder mixture can subsequently be heated for a certain period of time (for example, approximately two hours) to the diffusion acceleration temperature thereof. The greatest possible material density can be produced in the workpiece by said measure. The diffusion acceleration temperature is dependent on the alloy and, for example, in tool steel is of an order of magnitude of approximately 1150° C. In contrast thereto, alloys consisting of molybdenum have a higher diffusion acceleration temperature of above 1800° C. and alloys consisting of copper have a diffusion acceleration temperature which is lower than 900° C. The diffusion acceleration temperature is preferably maintained for a relatively long period of time, customarily for a period of time of more than 30 minutes. The maintaining time which, if appropriate, may also be a number of days depends in particular on the diffusion acceleration temperature and the pressure exerted. It can also be provided that the metal-containing material is at least partially melted and is compacted in an at least partially liquid state. The metal-containing material does not need to be completely melted. There is, for example, the option of melting only one phase of the metal-containing material before the compaction operation. This embodiment of the method may be advantageous for a number of application purposes.

There is, for example, also the option for at least two layers or regions having different metal-containing powders or metal-containing powder mixtures to be placed into the heat-resistant mold 2. A build-up of layers with at least two layers permits the production in a particularly advantageous manner of a workpiece (for example a shaping tool or part of a shaping tool) with graduated tool properties with the aid of the method presented here. It is thus possible, for example, to produce shaping tools or parts of shaping tools having different mechanical and/or physical properties within the volume thereof. A graduation of properties in the volume can be produced (continuously or discontinuously) in one, two or all three directions in space. A comparatively hard and wear-resistant tool surface is frequently desired whereas a tool base body which is softer by comparison is sufficient or may even be particularly advantageous.

In order to reduce the costs for the shaping tool or the part of a shaping tool, a material composition which is layered or differs in regions is likewise advantageous. The optimum tool properties which are normally associated with high costs can thus be provided only where they are actually required. The remaining part of a shaping tool or part of a shaping tool can be constructed from a material having sufficient properties and substantially lower material costs. It can furthermore be provided that the process heat is removed in a targeted manner with the aid of the optionally provided cooling device after the compaction of the metal-containing material. One purpose of such a targeted removal of heat can consist in the overall acceleration of the production method. Furthermore, by means of the targeted removal of heat, the microstructural properties of the workpiece can be set. In this case, it is particularly advantageous if the pressure is maintained during the targeted removal of the process heat (cooling phase). As a result, deviations in geometry, in particular shrinkages of the workpiece, can be substantially prevented in an advantageous manner. The cooling device may comprise, for example, a number of cavities which are introduced in a defined manner into the heat-resistant mold 2. A liquid or gaseous cooling fluid can flow through the cavities and can be conveyed by means of a supply device in order to be able to cool the metal-containing material in the heat-resistant mold 2 in a targeted manner.

The cooling device may also comprise, for example, at least one tube which is embedded into the heat-resistant mold 2 and through which a liquid or gaseous cooling fluid can circulate. The evacuable chamber 1 can furthermore also be flooded with the cooling fluid (for example, with nitrogen or argon). The gaseous cooling fluid can preferably flow out of a pressure tank or a compressed gas cylinder into the cavities, the tube or the evacuable chamber 1 since the gas is additionally cooled further upon expansion. There is in particular the option for the cooling device to form a cooling circuit within which the cooling fluid can circulate and within which, for example, a heat exchanger or a compression stage can be provided. 

1-20. (canceled)
 21. A method for producing a workpiece, which comprises the following steps: providing a heat-resistant mold having a first mold part and at least one second mold part in an evacuable chamber, the heat-resistant mold being formed of concrete, cement, or mortar admixed with at least one ceramic material; placing a metal-containing material into the heat-resistant mold; generating a vacuum in the evacuable chamber; heating the metal-containing material to form a heated material; and compacting the heated material in the heat-resistant mold by hot pressing under vacuum conditions.
 22. The method according to claim 21, which comprises carrying out a plurality of flushings of the evacuable chamber with one of a reduction gas or an inert gas prior to generating the vacuum in the evacuable chamber.
 23. The method according to claim 21, which comprises generating a high vacuum in the evacuable chamber and hot-pressing the heated metal-containing material under high vacuum conditions.
 24. The method according to claim 21, which comprises placing the metal-containing material in the form of at least one layer of a metal-containing powder or of a metal-containing powder mixture into the heat-resistant mold.
 25. The method according to claim 21, which comprises carrying out at least one of the following further process steps: carrying out a cold-pressing step prior to heating the metal-containing material; and at least partially melting the metal-containing material and compacting in an at least partially liquid state; and removing the process heat in a targeted manner after compacting the metal-containing material.
 26. A method of producing a workpiece, which comprises the following steps: providing a heat-resistant mold having a first mold part and at least one second mold part in an evacuable chamber, the heat-resistant mold being formed of concrete, cement, or mortar admixed with at least one ceramic material; placing a metal-containing material into the heat-resistant mold; generating a vacuum in the evacuable chamber and maintaining the vacuum for a period of time t_(vacuum); generating an inert gas atmosphere or a reduction gas atmosphere in the evacuable chamber after an expiration of the period of time t_(vacuum); heating the metal-containing material to form heated material; and compacting the heated material in the heat-resistant mold by hot pressing in the inert gas atmosphere or reduction gas atmosphere.
 27. The method according to claim 26, which comprises placing the metal-containing material in the form of at least one layer of a metal-containing powder or of a metal-containing powder mixture into the heat-resistant mold.
 28. The method according to claim 27, wherein the placing step comprises laying the metal-containing powder or the metal-containing powder mixture in at least two layers or regions having a different chemical composition.
 29. The method according to claim 27, which comprises carrying out at least one of the following further process steps: carrying out a cold-pressing step prior to heating the metal-containing material; and at least partially melting the metal-containing material and compacting in an at least partially liquid state; and removing the process heat in a targeted manner after compacting the metal-containing material.
 30. A device for producing a workpiece, comprising: an evacuable chamber; a heat-resistant mold disposed in said evacuable chamber, said mold having a first mold part and at least one second mold part together forming a mold cavity for receiving a metal-containing material, said heat-resistant mold being formed of concrete, cement, or mortar admixed with at least one ceramic material; a device for generating a vacuum in said evacuable chamber; a device for heating said metal-containing material in said heat-resistant mold; and a device for compacting the heated, metal-containing material in said heat-resistant mold by hot pressing.
 31. The device according to claim 30, wherein said mold cavity is configured to receive a metal-containing powder or a metal-containing powder mixture, and to form a shaping tool or a part of a shaping tool.
 32. The device according to claim 30, which further comprises a system for generating at least one of an inert gas atmosphere or a reduction gas atmosphere.
 33. The device according to claim 30, wherein said device for heating the metal-containing material comprises at least one heating element embedded in said heat-resistant mold.
 34. The device according to claim 30, which further comprises at least one cooling device for cooling the metal-containing material within said heat-resistant mold in a targeted manner.
 35. The device according to claim 34, wherein said cooling device is embedded in said heat-resistant mold.
 36. The device according to claim 30, wherein the concrete, cement or mortar of said mold contains at least 40% Al₂O₃.
 37. The device according to claim 30, wherein the concrete, cement or mortar of said mold has a strength greater than 150 MPa.
 38. The device according to claim 30, which further comprises a mechanically prestressed reinforcing ring for producing pressure stresses in the heat-resistant mold.
 39. The device according to claim 30, wherein at least sections of the surface of the mold cavity of said heat-resistant mold have a ceramic layer and/or a releasing-agent and lubricating layer.
 40. The device according to claim 30, wherein said device for compacting the metal-containing material comprises a metal cylinder operatively connected to said second mold part of said heat-resistant mold. 